This invention relates to a method and apparatus for purifying technetium-99m radiopharmaceuticals.
Technetium-99m radiopharmaceuticals such as .sup.99m Tc-methylene diphosphonate (MDP), .sup.99m Tc-diethylenetriaminepenaacetic acid (DTPA) and .sup.99m Tc-labeled proteins such as human serum albumin, tend to become contaminated with two radiochemical impurities, pertechnetate ions and hydrolyzed reduced technetium. These impurities detract from image quality and result in unnecessary radiation exposure to the patient. It is routine practice to test these radiopharmaceuticals after preparation for the presence of impurities, and if the impurity levels are found to be excessive, the lot or kit is discarded and a fresh lot or kit is prepared. Most of the commonly used technetium radiopharmaceuticals have a limited shelf life, usually three to six hours, because of the formation with time of excessive levels of these radiochemical impurities. Rejection of radiopharmaceuticals, either at time of preparation or before they are utilized, because of excessive impurity levels, is a major expense and waste of radioactive .sup.99m Tc. For this reason, it is highly desirable to maintain these compounds as free as possible from impurities.
In compounding of Tc-labeled compounds, stannous ions are used to reduce the pertechnetate (Tc VII) to technetate (Tc IV). The Tc IV species forms complexes with various ligands, i.e., MDP, DTPA, proteins, etc. The complexes of Tc constitute the radiopharmaceutical or diagnostic radioactive tracer. On standing, dissolved oxygen in the formulation causes the stannous ions to become oxidized to stannic ions and as the concentration of stannous is reduced by the spontaneous oxidation, the Tc-complex begins to oxidize, converting the Tc IV back to Tc VII and releasing it from the complex and forming the radiochemical impurity. The presence of excess stannous ions in the formulation tends to minimize reoxidation of the technetium. Excess stannous ions, however, can enhance the formulation of reduced hydrolyzed technetium, a form of technetium which is insoluble and unavailable for complex formation.
Even by carefully controlling the concentration of stannous ions in these formulations and utilizing other antioxidants, technetium-labeled radiopharmaceuticals must be utilized in conventional diagnostic procedures very shortly after preparation because of the deterioration of these compositions due to oxidation. It would be highly desirable to provide a means for reactivating or purifying technetium radiopharmaceuticals that have been degraded by oxidation.